1. FIELD OF THE INVENTION
The field of the invention is ultrasonic thermometers of the pulse-echo type and electronic apparatus for generating the electrical pulse and processing the reflected acoustical pulse to obtain temperature information.
2. DESCRIPTION OF THE RELATED ART
Ultrasonic thermometers operate on the principle that the velocity of sound in any medium is a function of temperature. In ultrasonic thermometers of the pulse-echo type, a transducer is used to produce pulsed acoustic signals which propagate down a sensor rod of a known material. Discontinuities are placed along the sensor rod at locations where temperature measurements are desired. An acoustic signal propagating down the sensor rod will reflect (in essence echo) part of its energy at these discontinuities. The time between adjacent reflections varies as a function of temperature. After calibration, temperature is obtained by measuring the time between adjacent reflections. By placing several discontinuities along the length of the rod, temperature profiles can be obtained.
Over the past several years it has been shown that ultrasonic thermometers are capable of measuring temperature in hostile environments that have precluded the use of conventional instrumentation such as thermocouples and optical pyrometers. The vast majority of this past work has been in research and development (R&D), primarily in nuclear reactor applications.
In order for the use of ultrasonic thermometers to extend beyond R&D and into industrial processing, significant improvements in accuracy, reliability, and cost must evolve through innovative developments in the field.
Some of the key areas where problems have arisen in the adaptation of ultrasonic thermometry to industrial application, and where solutions are provided by the Applicants' invention, are reviewed below.
Industrial processing is well known for its high demand for electrical power, large processing equipment, and lack of ambient temperature control. Power is commonly controlled by SCR's (Silicon Controlled Rectifiers) which, when turning on and off, create radiated interference as well as large electrical spikes on the power line. This constitutes noise which can interfere with the electronic apparatus utilized in conjunction with the ultrasonic thermometers. The large size of industry's processing equipment generally means that the ultrasonic thermometer sensor rod must be longer and that the electronic signal processing apparatus must be located at distances far away from the unit where temperatures need to be measured. This results in attenuation of the signal information which can also greatly affect the ultrasonic signal processing apparatus. Further, the lack of ambient temperature control can cause the electronic apparatus to drift resulting in error in its temperature output.
Prior ultrasonic thermometer improvements have either given little or no attention to noise problems or have employed standard noise elimination techniques such as shielding, grounding antennas, or isolating power sources. These standard techniques are not enough in industrial environments since SCR noise can be picked up by the ultrasonic sensor rod (shielded or unshielded) inside the heating unit through radiation.
In addition, prior ultrasonic thermometer inventions have either failed to compensate for errors associated with signal attenuation or improvements which have been developed have proven complex, costly, or susceptible to errors from deviations in power line voltage, ambient temperature, and other effects normally encountered in industrial environments. More specifically, prior ultrasonic thermometer electronic processing equipment utilized for time interval measurements can be classified into two general categories: (1) Peak detection systems, a discussion of which can be found in U.S. Pat. No. 3,717,033, Gordon et al. Ultrasonic Apparatus, Particularly for Thermometry, 1973, and (2) Zero Cross detection system with tracking servos, a discussion of which can be found in Sandia Report No. SAND-79-0621, G. A. Carlson, et al. "An Ultrasonic Thermometry System For Measuring Very High Temperatures in Reactor Safety Experiments."
Peak detection systems become erroneous when the peaks of the information signals vary inconsistently. Such cases are frequently encountered when attenuation occurs as a result of temperature changes along the length of the sensor line. Attenuation can change the information signals even though the temperature around the sensor location where the information signals are produced may not have changed. Changes in the information signals change the peak references and therefore produce error. This effect is of particular concern in industrial environments where the sensor line must pass through several heating zones which are independently controlled. Zero Cross detection systems with tracking servos are quite independent of signal amplitude and are therefore less susceptible to error from signal attenuation. Tracking servos, however, are inadequate for industrial application of the ultrasonic thermometer. Tracking servos, besides being costly, have a feedback loop which must be zeroed to room temperature in order to maintain a consistent tracking output. If the tracking servo loses its reference, such as would be the case during power failure or the presence of a large noise spike, the tracking servo can not be reset without introducing an error unless the system is cooled down to room temperature. It also can not be reset without the use of an oscilloscope. This is obviously impractical in industrial processing. Furthermore, tracking servos are susceptible to drift due to changes in ambient temperature. It is recognized that such drift may be prevented by the use of elaborate protection mechanisms such as ovenized enclosures but this type of solution is more than likely impractical from cost considerations.